1. Field of the Invention
This invention relates to an audio signal reproducing apparatus and, more particularly, to a small sized audio signal reproducing apparatus in which reproduced audio signals may be heard by a headphone.
2. Description of the Prior Art
Recently, a small sized audio signal reproducing apparatus, in which audio signals may be heard by a headphone, has become popular to provide for a more extensive range of appreciation of music or the like.
With the small sized audio signal reproducing apparatus for hearing the reproduced audio signals with a headphone, there is a demand for further reduction in size and weight since such reduction possibly leads to more widespread use and mode of application.
However, with the above described small sized audio signal reproducing apparatus, magnetic tapes or optical disks, such as, for example, compact disks, are used as the signal recording media, so that the apparatus in its entirety cannot be reduced in size beyond the size of these recording media. On the other hand, since the mechanical parts for driving the recording media are necessitated, the operational reliability may be lowered due to wear or damage to movable parts. The playback operation may also be affected by mechanical disturbances, such as vibrations. In addition, there is a limit to reduction in size and weight due to the use of an electrical motor or a plunger.
Recently, with the progress in the technique of high efficiency compression encoding of audio-signals, high-fidelity playback sounds may now be produced even at the rate of the order of, for example, 64 kb per second per channel. Among the techniques for such high efficiency compression encoding, there are a sub-band coding (SBC) in which audio signals on the time axis are divided into a plurality of frequency bands prior to encoding; an adaptive transform encoding (ATC) in which signals on the time axis are converted by orthogonal transform into signals on the frequency axis which are then divided into a plurality of frequency bands, and adaptive encoding is performed in each of these frequency bands; and an adaptive bit allocation (APC-AB), which is a combination of the above described SBC and an adaptive predictive encoding (APC) and which consists in dividing the signals on the time axis into a plurality of bands, converting the band signals into base-band signals or low-frequency signals and performing plural order linear predictive analyses for predictive encoding.
For compression encoding of audio signals on the left and right stereo channels, it has now become possible to transmit high fidelity stereo audio signals at the transmission rate in the order of 64 kb per second per channel through the use of the correlation between the left and right stereo signal or the use of codes of unequal lengths.
It is therefore an object of the present invention to provide a headphone type ultra small size audio signal reproducing apparatus in which high quality audio signal reproduction may be achieved with the small data volume through the use of high efficiency compression encoding and a semiconductor memory as a signal recording or storage medium.
For accomplishing the above object, the present invention provides a headphone type audio signal reproducing apparatus comprising a semiconductor memory for storing digitized and high efficiency compression encoded audio signals, a decoder for reading out data stored in said semiconductor memory and decoding the read-out data, by way of performing an operation which is an inversion of compression encoding, a digital/analog converter for converting output signals from said decoder into analog signals, and a headphone unit for converting output signals from said digital/analog converter into acoustic signals.
As a typical example of the high efficiency compression encoding operation, input signal signals are divided into a plurality of frequency bands so that the bandwidths will be broader for progressively higher frequency bands, the allowable noise level is set on the band-by-band basis in accordance with the energy of each band and the components of each band are quantized with the number of bits corresponding to the level of the difference between the energy of each band and the preset allowable noise level. In presetting the allowable noise level, the allowable noise level is set so as to be higher for the same energy for progressively higher frequencies for minimizing the degradation in the sound quality and reducing the bit rate. In this manner, the requirements for satisfactorily reproducing audio signals with the use of semiconductor memories, that is, the requirements for reproducing so-called high fidelity level audio signals for at least several minutes, may be satisfied.
With the reproducing apparatus of the present invention, which makes use of the semiconductor memory as the signal recording medium, to reduce the size and weight of the apparatus drastically so compared to the reproducing apparatus which makes use of the magnetic tape or optical disk as the recording medium.
That is, since the semiconductor memory is used as the recording medium for high efficiency encoded audio data, not only may the main body of the reproducing apparatus be reduced in size and weight, but the movable parts, such as are necessitated in the conventional reproducing apparatus in which disks or tapes used as recording medium are driven mechanically for reproducing the audio signals, may be eliminated, so that the apparatus superior in service life or in durability may be obtained.
In addition, the masking level is raised for higher frequencies during data compression to reduce the number of bits allocated at the time of quantization to realize the bit allocation suited to the human auditory sense. Thus the degradation in the sound quality may be minimized with the use of a lesser number of bits or a lower bit rate so that the audio signal reproduction may be made satisfactorily with the use of the semiconductor memory chips.